Hematopoiesis is the formation of blood cells that occurs in the bone marrow, spleen, liver, and lymph nodes. All blood cells (erythrocytes, platelets, white blood cells, etc.) ultimately originate from hematopoietic stem cells (HSCs). Through differentiation into different progenitor cells, HSCs have the unique ability to give rise to all of the different mature blood cells. This property has led to intense efforts to isolate, produce, and culture HSCs, as these cells would have great use in the treatment of a wide variety of diseases and disorders.
HSCs give rise to all blood cell types in an adult and maintain this ability after asymmetric division. To maintain such self-renewal throughout the life of an organism, HSCs are often quiescent and found in specialized microenvironments or niches of the bone marrow. A number of HSC niche models and soluble signals that regulate HSC maintenance have been described (Kiel et al., 2008, Nat Rev Immunol, 8(4): 290-301), but there remains a need to understand physical aspects of the niche, including forces imposed or generated by HSCs. Externally imposed shear stresses are known to regulate embryonic hematopoiesis (Adamo et al., 2009, Nature, 459(7250): 1131-1135), and extensional elastic properties of a tropoelastin matrix reportedly influence HSC expansion (Holst et al., 2010, Nat Biotechnol, 28(10): 1123-1128). However, it remains unclear what molecules integrate physical attributes of matrices and microenvironments to mechanically regulate HSC fates.
Platelets, or thrombocytes, are small cellular fragments which are important for the formation of blood clots. These platelets are formed through the fragmentation of megakaryocytes (MKs). MKs are large blood cells found within the bone marrow and are characterized by having a unique lobed structure and by having several copies of cellular DNA. These unique properties of MKs occur because these cells replicate their DNA without cytokinesis (division) in a process called endomitosis. Once a MK matures, it is fragmented into many platelets which go into the blood circulation. Reduced platelet count, known as thrombocytopenia, can occur in a number of disease processes. Subjects with reduced platelets can have trouble clotting, have abnormal bruising, and experience general fatigue and weakness.
Despite research efforts, there is currently no known strategy to cheaply and easily isolate and culture rare blood cells, including HSCs and MKs. Thus, there is a need in the art for a bioreactor that can isolate rare blood cells, and for methods that isolate, use and assay rare blood cells. The present invention addresses these unmet needs in the art.